Observations of pipe-soil response from in-situ measurements

Abstract

Abstract The force mobilized through the interaction between a pipeline and the seabed is a key to the design of pipelines. Fugro Smartpipe® is a site investigation tool that is designed to measure pipe-soil interaction forces in-situ, at the seabed and at close to full-scale. The tool has been deployed at several offshore sites and valuable data has been collected. This paper presents some results and observations from recent testing offshore West Africa and focuses mainly on (i) the vertical pipe penetration tests, (ii) the dissipation of excess pore pressures generated during vertical penetration, and (iii) the subsequent axial pipe-soil response. In particular, the effect of pipeline unloading between the consolidation phase and the axial testing phase is examined in terms of the pore water pressure response at the pipe surface and the measured axial friction factor. The results indicate that the use of a drained friction factor could be justified if the pipeline is overloaded by a factor of the order two. Such pre-loading is quite typical for light pipelines that are hydrotested or installed in waterfilled conditions. Introduction Deep water pipelines are generally operated under high internal temperature and pressure, which can cause pipeline end expansion, cyclic axial walking and lateral buckling. The interaction force between a pipeline and the seabed is a key factor for pipeline design (Bruton et al, 2008). Considerable capital expenditure can be saved by relatively small adjustment in the magnitude of expected interaction force. Such saving stems from reduced requirement for stabilisation and anchoring of the pipeline and the reduced need to tolerate end expansion (Hill and Jacob, 2008). Fugro Smartpipe® is a site investigation tool that is designed to measure pipe-soil interaction forces in-situ, at the seabed, at close to full scale. The equipment comprises a seabed frame with an instrumented model pipe that can be driven in the vertical, axial and lateral directions whilst the corresponding loads and associated excess pore pressures are recorded. The merit of Smartpipe, compared with other techniques, is that it measures the pipe-soil interaction in undisturbed soil conditions at the seabed surface, at close to full scale. Smartpipe has been deployed at several offshore sites in West Africa and Australia and valuable data has been collected. Some results and observations from the first offshore campaign were published by White et al (2010). The current paper presents some results and observations from a recent campaign offshore West Africa. This paper presents data on (i) vertical pipe penetration tests, (ii) dissipation of excess pore water pressure adjacent to the pipe that was generated during vertical penetration, and (iii) the subsequently measured axial pipe-soil interaction. In particular, the effect of pipeline unloading between the consolidation phase and the axial testing phase is examined in terms of the pore water pressure response at the pipe surface and the measured axial friction factor. The range of tests covers the typical loading sequence that would be experienced by light pipelines that are hydrotested or installed in waterfilled conditions.